In a typical heating and air conditioning system for a vehicle, such as an automobile or a truck, air is circulated into the cab of the vehicle by a blower, driven by an electric motor. A blower switch is generally provided, having several positions to regulate the speed of the motor. Thus, low, medium and high speed positions may be provided. The air circulated by the blower may be heated by a heater coil, or cooled by an air conditioner cooling coil. The air conditioner includes a compressor to compress the refrigerant, which is then cooled by a condensor coil and supplied to the cooling coil. The compressor is usually controlled by an electrically operable drive clutch, connected between the engine of the vehicle and the compressor. An air conditioner switch is generally provided for energizing the air conditioner by energizing the clutch, so that the clutch drives the compressor.
It has been found that a minimum amount of air should be continuously circulated through the cooling coil of the air conditioner, whenever the compressor is energized, to prevent frost from building up on the cooling coil. The frost results from condensation and freezing of moisture, condensed from the air. If a sufficient amount of air is circulated through the cooling coil, the air supplies sufficient heat to the cooling coil to prevent the buildup of frost. If the amount of air circulated through the cooling coil is not sufficiently great, frost can build up on the cooling coil until the cooling coil is completely clogged by the frost, so that the air conditioner becomes completely inoperative.
To prevent the buildup of frost, it has been common practice to keep the blower running at all times when the ignition switch of the vehicle is turned on. For this arrangement, the blower control switch does not have an OFF position, although the switch may have several different speed positions, such as low, medium and high. At the very least, the blower motor is kept operating at a low speed, so that the desired minimum amount of air is always circulated through the cooling coil of the air conditioner.
The continuous energization of the blower motor has the disadvantage of being wasteful of energy. Moreover, the blower causes a constant draft, which may seem uncomfortable to some persons under some conditions.
To overcome these disadvantages, it would be desirable to provide a blower switch having an OFF position, so that the blower could be deenergized and stopped completely, while providing some other means to prevent any possible frost buildup on the cooling coil of the air conditioner. One approach has been to provide extra electrical contact means in the air conditioner switch, and extra electrical wiring between the air conditioner switch and the blower motor, to be sure that the blower motor will be energized, at least at a low speed, so that a minimum flow of air will be circulated through the cooling coil whenever the air conditioner switch is moved to its ON position, so as to energize the air conditioner. This approach has the disadvantage of adding substantially to the cost of both the air conditioner switch and the electrical wiring.